1. Field of the Invention
The present invention relates to an apparatus for cooling samples or wafers at a cryogenic temperature so as to perform microscopic observation, microspectroscopic analysis, near-field microscopic observation, near-field spectroscopic analysis, and photoconduction characteristic evaluation, or conduction characteristic evaluation.
2. Description of the Related Art
When samples or wafers are cooled at a cryogenic temperature with a refrigerating machine, a vibration of the refrigerating machine as a vibration source propagates to the samples or wafers. As a result, the samples or wafers vibrate or drift with respect to a measurement reference surface, which deteriorates a precision of microscopic observation, microspectroscopic analysis, near-field microscopic observation, near-field spectroscopic analysis, photoconduction characteristic evaluation, or conduction characteristic evaluation.
Conventionally, a method for connecting a vacuum vessel and a refrigerating machine by way of a vibration reducing adapter has been proposed in order to prevent samples or wafers from vibrating or drifting as disclosed in Japanese Patent laid-open publications No. Tokkaihei 05-243042, No. Tokkaihei 07-84058, No. Tokkaihei 09-50910, No. Tokkaihei 11-87131 and No. Tokkaihei 11-512512. The vibration reducing adapter can decrease the vibration or drift of the samples or wafers. It is still insufficient for an optical measurement in a high spatial resolution. In association with such a method, a method for reducing vibration with a weight attached to the refrigerating machine as disclosed in Japanese Patent laid-open publications No. Tokkaihei 09-229997 and No. Tokkai 2006-41259, and a method for reducing vibration by securely fixing the refrigerating machine on a floor as disclosed in Japanese Patent laid-open publication No. Tokkai 2005-24184 have been proposed. These methods can further reduce vibration or drift of samples or wafers but are insufficient for the optical measurements requiring a high spatial resolution of 1 micrometer or less.
In Japanese Patent laid-open publication No. Tokkai 05-245395, there is proposed a method for nulling a relative motion between a sample and a measurement system by fixing the sample and the measurement system to a vibrating potion of the refrigerating machine. However, the relative motion cannot become zero as a matter of fact, because the vibrating portion of the machine vibrates acceleratingly which makes the measuring system or the member fixed to the measuring system to deform.
Furthermore, a method for cooling a sample with air, helium gas, or liquid helium so as not to propagate vibration of the refrigerating machine directly to the sample is known in the art as disclosed in Japanese Patent laid-open publications No. Tokkaihei 05-297092, No. Tokkaihei 06-74819 and No. Tokkaihei 06-109821. However, the substantial vibration reducing effect is insufficient, because the vibration of the refrigerating machine propagates indirectly via a sample supporting member. A method for stopping the refrigerating machine once during the measurement is known in the art as disclosed in Japanese Patent laid-open publication No. Tokkai 2002-277086. However, this method is unsuitable for the precision measurement, because of unstable temperatures and a large drift of a sample.